The present invention relates to a method of reconditioning an electronically programmable memory device to re-open its Write/Erase window after it has narrowed significantly from repeated use.
Devices such as electrically erasable programmable silicon semiconductor read only memory units start out with a Write/Erase window of the order of 5 volts with about a 3.5 volt threshold in the programmed or write mode and about a -1.5 volts threshold in the erase mode. After many read and write cycles this window begins to be reduced. For example, after about 100 kilocycles this window may be only 2 volts and after several thousand more cycles may disappear altogether. It is thought that this gradual reduction in the window may be due to trapping of charge in the silicon dioxide at the silicon-dioxide interface due to the breaking of oxygen bonds in this region. The broken oxygen bonds serve to trap electrons. A small window makes it difficult to distinguish between a "1" and a "0".
It is therefore a principal object of this invention to provide a method of extending the life of an electrically erasable programmable read only memory device. In particular, it is an object to provide a method of reconditioning such a device after its Write/Erase window has been reduced significantly so as to re-open its window.